1. Field of the Invention
This invention relates generally to liquid development of latent images on electrographic or electrophotographic recording members. More particularly, the invention relates to a liquid development of latent images produced on a movable image retention belt for high speed reproducing machines by using a moving belt applicator to define a development zone having a uniform gap with an extended length.
2. Description of the Prior Art
Electrographic printers form electrostatic latent images on a dielectric surface of a conductive belt of drum by means of an ion deposition process. This process records the informational areas contained on an original document or encoded in digital form. The latent image is developed by bringing a developer material in contact with it. The developer consists of a liquid carrier containing dispersed pigmented toner particles. The toner particles are deposited in image configuration onto the dielectric surface of the conductive member, after which the developed image is transferred to a copy sheet. After transfer, heat is applied to the copy sheet to permanently fuse the toner particles, and the dielectric surface of the conductive member is cleaned for another electrostatic latent image. In electrophotographic printers, an electrostatic charge is placed uniformly over a photoconductive surface of a conductive belt or drum and the charge photoconductive surface is moved past an exposure station, where a flowing light image of an original document or encoded digital information is imaged thereon. An imaging operation at the exposure station selectively dissipates the charge on the photoconductive surface in the light exposed region, thereby recording the original document or digital data thereon in the form of a latent electrostatic image. The remainder of the electrophotographic printing process is similar to the above-described electrographic printing process. Existing methods for developing latent images of charged areas on image retention members with liquid developers consist generally of two types; namely fountain developers and rolling cylinder developers. Fountain developers force a liquid developer through a small channel up to a lip spaced from an image retention member to form a development gap height. The liquid developer is then returned to a liquid sump by gravity or vacuum. Many variations of this fundamental configuration exist, but they suffer from a number of difficulties. The channels through which the liquid developer must pass are necessarily quite thin and they are subject to being narrowed and even blocked by liquid developer adhesion to the walls of the channel. This condition would be further aggravated if a developmental bias is used to reduce background noise in the developed image. Development of latent images in high speed printers or reproducing machines would require many channels through which the liquid development would be forced to flow the liquid developer against the latent images.
Rolling cylinder developers pull liquid developer into the development gap by surface tension and viscous drag. The only narrow channel is in the development gap between the rolling cylinder and the image retention member. A development bias voltage may be effectively employed to reduce background, but the large development gap at the leading edge of the meniscus introduces a much weaker field than exists at the narrowest part of the developmental gap. This variation in the gap distance adversely influences the development of the latent image. The length of the development gap is limited by the geometry of the rolling cylinder and the image retention member. For high speed applications, many such rolling cylinders must be used in tandem.
A significant problem encountered in liquid development apparatus is that the developer comprises a liquid carrier with entrained toner particles and the liquid developer in contact with latent images tend to become void of toner particles in the vicinity of the latent images. This may occur even after a very short period of time, since the concentration of toner particles in the liquid near where the developer is brought into contact with the image retention member may be lowered very rapidly when developing a latent image having a large solid area. As processing speeds for electrographic and electrophotographic machines increased, liquid development apparatus required modifications to keep pace with the increased throughput. It was found that as the image retention member was passed over a rotating cylindrical developer, the developing zone was relatively small and that effective development was limited by the volume of liquid developer that the rotating cylindrical developer could deliver to the processing nip. Similarly, for high speed development, the fountain developers required many more channels through which the ink was forced to flow in a fountain-like fashion.
U.S. Pat. No. 4,907,532 to Mikelsons et al. discloses a development apparatus for the application of liquid toner to the surface of a cylindrical electrophotographic image receptor bearing a latent image. A liquid development apparatus comprising an endless belt electrode is tensioned between two rollers and positioned to be driven orthogonally with respect to the movement of the receptor. The geometrical positioning of the belt with respect to the receptor produces a developmental zone defined by the width of the endless belt electrode and the gap between the belt electrode and the receptor. In this configuration, the endless belt electrode is parallel with the axis of rotation of the cylindrical receptor and perpendicular to the direction of rotation of the receptor. In an alternate embodiment, the span of belt electrode forming the development zone is magnetically shaped in a base support member to provide a uniform development gap between the belt electrode and the cylindrical receptor. Liquid toner from a continuous supply of toner is forced into the development zone by supply channels and return channels. In one embodiment, the supply channel is a series of holes spaced along the base support member and slots are formed in the endless belt electrode to allow the liquid toner to flow into the development zone between the belt and the cylindrical receptor. The liquid toner is extracted from the development zone be means of return passageways on either side of the belt. Extraction of the liquid development is accomplished by a source of vacuum which pulls the liquid developer into and through the return passageways.
U.S. Pat. No. 4,410,260 to Kuehnle discloses method and apparatus for liquid development using an endless electrophotographic belt with a photoconductive surface on its exterior surface. The belt is mounted on two rollers and the bottom reach is charged and imaged as the belt moves in one direction. When the belt passes the development station, a cylindrical development roller applies liquid toner from a sump to the latent image. The development roller protrudes into the belt causing it to wrap around a portion of the periphery of the development roller for the purpose of producing a narrow gap between the belt and the development roller. The purpose of the electrophotographic belt wrapping around a portion of the development roll is to extend the development zone. However, unless annular collars are used to provide a uniform spacing of the electrophotographic belt from the development roll the development zone would be a complex function of the relative speed of the two elements, the compliance of the electrophotographic belt, the tension of the electrophotographic belt, and the viscosity of the developer. Thus, without the annular collars the layer of developer about the surface of the development roll would not help a uniform thickness along the axis of the development roll.
U.S. Pat. No. 4,883,018 to Sagiv discloses a fountain-type liquid development system to develop latent images on a planar image retention member. The image retention member is a flexible belt that moves in a direction opposite to the direction of gravitational forces exerted on it. An extended development zone is formed such that it is parallel to the surface possessing the latent image. The liquid developer material is pumped from a sump into the development zone and then circulated back to the sump.
U.S. Pat. No. 4,796,048 to Bean discloses a resilient intermediate transfer member and apparatus for liquid ink development. A conventional liquid development system develops latent images on an electrophotographic drum and the developed images are transferred to an intermediate belt by a biased transfer roll. The developed images are then transferred from the intermediate belt to a copy sheet which is then passed through a fuser to permanently fix the developed images thereon.
U.S. Pat. No. 4,918,487 to Coulter, Jr. discloses method and apparatus for applying liquid toner to very small areas of a recording surface used in high speed micro imagery applications. The method includes a photoconductor disposed over a conductive substrate and having at least one fractional area containing a latent image that is to be developed. A flexible belt having a dielectric layer on its own conductive substrate acts as a carrier mechanism. Liquid toner is electrostatically deposited on the carrier member to form a predeposit of desired density in an area corresponding to the fractional area to be developed on the photoconductor. The transfer of toner is accomplished by a pair of plungers. When engaged, contact is made between the predeposit on the flexible belt and the fractional area on the photoconductor containing the latent image.